Recently, semiconductor devices that are capable of transmitting and receiving data without contact have been actively developed. Such semiconductor devices are particularly called RFID (radio frequency identification) tags, ID tags, IC tags, IC chips, RF tags, wireless tags, electronic tags, wireless chips, transponders, and the like.
The data transfer between the RFID tag and a communication device is generally carried out in such a manner that a device on the transmission side (e.g., the communication device) transmits a modulated carrier wave, and the device on the reception side (e.g., the RFID tag) demodulates the modulated carrier wave.
As one of the methods for modulating a carrier wave, there is an amplitude modulation method (an amplitude shift keying (ASK) modulation method). An ASK modulation method is a method in which a difference in amplitude of a carrier wave is generated and this amplitude difference is used as a modulation signal in transmitting data.
In order to extract data from the modulated carrier wave, the RFID tag is provided with a demodulation circuit. As an example of the demodulation circuit, there is a device that uses a rectifying function of a transistor whose gate terminal and drain terminal (or source terminal) are connected to each other (e.g., see Patent Document 1).
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